The present invention relates to antennas for measuring electromagnetic wave field quantities and in particular to antenna arrays having an arrangement of antenna elements with specific dimensions, spacing and impedance for improved performance.
In recent years there has been a widespread proliferation of wireless communication systems such as cellular telephones or short range microwave communication links replacing computer network cables in so-called Bluetooth systems. However, with this proliferation there is much concern throughout the world that radiation from electromagnetic fields causes human body tissue damage. The antenna and the body of wireless electronic equipment such as cellular telephones come into close contact with a person""s head or other sensitive body tissue, thereby creating a close exposure to an electromagnetic field. Therefore, there is a growing demand to maximize performance of the wireless communication systems while simultaneously minimizing exposure of the human body to electromagnetic fields.
In order to meet this demand it is necessary to measure the electromagnetic field components in the near xe2x80x94field of radiating sources. Examples of such measurements are the characterization of near-field distributions of antennas and oscillators, evaluation of electromagnetic interference emissions, measurement of near-field quantities in bioelectromagnetic phantoms, measurement of scattered fields from microwave medical imaging, etc.
A major difficulty is the measurement of complex electromagnetic field distributions over larger areas or in larger volumes typically having dimensions between 1 and 100 wavelengths, to be measured with sampling intervals being a portion of a wavelength. For such measurements, single antenna mechanical scanning of a volume under test, disclosed for example in U.S. Pat. No. 5,585,808 issued to Wysome in Dec. 17, 1996, U.S. Pat. No. 6,188,365 issued to Mattsson et al. in Feb. 13, 2001, or in PCT Application WO 01/75460 A1 published Oct. 11, 2001, requires many hours of testing to be repeated upon modification of any test condition, which are incorporated herein by reference. In addition, the use of a moving single probe for measuring the electromagnetic field distribution within a volume is inherently crude.
Use of preferably, multi-dimensional fixed antenna arrays provides a substantial reduction of measurement time while providing the potential for an increased accuracy of the
However, measurements using fixed antenna arrays suffer from three problems not present in single antenna systems. The dipole elements forming the antenna array are typically made of highly conducting materials, causing reflection and refraction of an incident electromagnetic wave at the boundaries of the array. From the finiteness of the array, edge effects typically occur and are undesirable. The reflections interact with other elements, in particular, a near-field radiating source. The refractions lead to errors in the field measurements. An electromagnetic wave incident on a dipole element of the antenna array induces a current in the dipole elements. The induced current causes re-radiation generating a scattered field which then induces a current in other dipoles of the antenna array.
Two-dimensional antenna arrays of terminated small dipoles and loops have been developed to reduce the problem of measurement time as disclosed in Collingnon, G., et al. xe2x80x9cQuick Microwave Field Mapping for Large Antennasxe2x80x9d, Microwave Journal, December 1982, pp. 129-130, and Previti, J. S., xe2x80x9cA Tool to Measure EMT Emissions from Printed Circuit Packsxe2x80x9d, EMC Testing, November 1989, pp. 4-5, references to which are incorporated herein by reference. However, no design criteria for antenna dimensions, spacing, and loading have been described and complex numerical calculations are required for each array configuration in order to ensure that each element of the array meets the conditions of low scattering, mutual coupling and edge diffraction.
It is, therefore, an object of the invention to provide a fixed antenna array for measuring of electromagnetic fields.
In accordance with the invention, there is provided an antenna array for measuring an electromagnetic field comprising: a plurality of substantially identical antenna elements having a substantially shorter length than a wavelength of the electromagnetic field and being placed at a substantially same spacing between consecutive antenna elements, the spacing being larger than approximately three times the length of the antenna elements, each antenna element for sensing the electromagnetic field at a predetermined location and for providing a signal in dependence thereupon, the signal being indicative of an approximately accurate electromagnetic field strength substantially absent mutual coupling effects between the antenna elements.
In accordance with the invention, there is provided an antenna array for measuring an electromagnetic field comprising: a plurality of substantially identical antenna elements having a substantially shorter length than a wavelength of the electromagnetic field and being placed at a substantially same spacing between consecutive antenna elements, the spacing being larger than approximately three times the length of the antenna elements, each antenna element for sensing the electromagnetic field at a predetermined location and for providing a signs in dependence thereupon, the signal being indicative of an approximately accurate electromagnetic field strength substantially absent scattering effects between the antenna elements.
In accordance with the invention, there is further provided an antenna for measuring an electromagnetic field comprising: an array of antenna elements for sensing the electromagnetic field at a predetermined location and for providing a signal in dependence thereupon; an electromagnetically separating layer for substantially shielding the electromagnetic field sensed by the at least an antenna element from electromagnetic radiation emitted from circuitry transmitting the signal; and, an absorber disposed between the electromagnetically separating layer and a source of the electromagnetic field for attenuating electromagnetic radiation.
In accordance with another aspect of the invention, there is provided a method for measuring strength of an electromagnetic field emitted from a source comprising the steps of: disposing at a predetermined location a plurality of substantially identical antenna elements having a length being substantially shorter than a wavelength of the electromagnetic field and having a substantially same spacing between consecutive antenna elements, the spacing being larger than approximately three times the length of the antenna elements; measuring a field strength of the electromagnetic field at each antenna element and providing a signal in dependence thereupon, the signal being indicative of an approximately accurate electromagnetic field strength substantially absent mutual coupling effects between the antenna elements; and, determining the approximately accurate electromagnetic field strength at each predetermined location absent a step of signal post processing for correcting the mutual coupling effects between the antenna elements.
In accordance with the other aspect of the invention, there is also provided a method for measuring strength of an electromagnetic field emitted from a source comprising the steps of: disposing at a predetermined location at least an array of antenna elements for sensing the strength of the electromagnetic field and for providing a signal in dependence thereupon; disposing an electromagnetically separating layer for substantially shielding the electromagnetic field sensed by the at least an antenna element from electromagnetic radiation emitted from circuitry transmitting the signal; disposing an absorber between the electromagnetically separating layer and a source of the electromagnetic field; and, measuring the strength of the electromagnetic field at the at least an antenna element and providing a signal in dependence thereupon, the signal being indicative of an approximately accurate electromagnetic field strength absent a step of signal post processing for correcting distortions of the measured field strength due to electromagnetic radiation emitted from the circuitry transmitting the signal.